Muvizu's activation server will only let you try to activate 10 times before marking your key as "used". This is to prevent piracy. Each licence key is only intended for use by one person (But can be used on multiple computers).
Therefore, you need to contact them asking for an extension to your activation attempts. It is up to them whether to do this. If they deny your request, you will need to purchase a new licence as explained above.
This is probably because of a change to your network device. Have you recently switched from a wired connection to a wireless one etc? This would change your MAC address and therefore Muvizu would not recognise the playplus.lic file as a match.
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